This invention relates to a method of and appartus for controlling a gas separation adsorption system during periods of reduced product withdrawl rate, i.e. "turndown".
Adiabatic pressure swing systems are used for separating feed gas mixtures by selective adsorption of at least one component in at least two adsorption beds and withdrawing one component-depleted product gas therefrom. While one adsorption bed is receiving feed gas at least one other bed is being depressurized, purged of the one component adsorbate and at least partially repressurized in preparation for return to the feed gas mixture treatment step. To provide a continuous supply of product gas the adsorbent beds are commonly arranged in parallel flow relationship and operated in a repetitive cycle.
Adiabatic pressure swing adsorption systems may by further classified in terms of variable adsorption pressure and constant adsorption pressure. In the latter the pressure in the adsorbent bed processing feed gas is substantially constant during the entire feed gas step, and this type of system is commonly used where the product consuming means requires product gas at constant and relatively high pressure. In the variable adsorption pressure system the feed gas mixture is introduced at progressively higher pressure to the inlet end of the adsorbent bed and depending on the product withdrawl rate, the bed pressure varies and most commonly increases during at least the first part of the adsorption step. The variable pressure adsorption system is best suited where the product consuming means do not require product gas at constant pressure and can accept relatively low pressure gas. This invention relates to only the variable pressure type of adiabatic pressure swing adsorption system wherein feed gas mixture is introduced at prograssively higher pressure to the inlet end of the adsorbent bed thereby progressively increasing the pressure thereon, and one component-depleted gas is discharged from the opposite or discharge end at variable pressure with at least part thereof withdrawn from the system as product gas.
In some product consuming means, the product gas withdrawl rate periodically and uncontrollably varies on demand over multiple cycles from a maximum rate to reduced rate substantially below the maximum rate. One such pressure swing adsorption system - product consuming means is an air separation adsorption system from which oxygen product gas is withdrawn and supplied as the aeration gas for a wastewater treatment system. The wastewater feed rate (and hence the oxygen demand rate) may vary greatly depending on seasonable or diurnal conditions.
The variable adsorption pressure system for such varying product demand rates must be designed to supply the maximum rate, since it is not usual practice to store product gas on-site. Nevertheless, the system must be capable of supplying product gas at reduced rates substantially below the maximum rate in a relatively efficient manner. By way of example, the percent recovery of product gas from the feed gas mixture at reduced product demand rate should not be greatly below the recovery at maximum or design rate. Moreover, the product gas purity at reduced product demand rate should be at least the specified average product purity. As used herein, "specified average product purity" means the average purity of product gas withdrawn from the pressure swing adsorption system per cycle at maximum product gas withdrawl rate.
Certain embodiments of the variable adsorption pressure type of adiabatic pressure swing adsorption system are described in Batta U.S. Pat. No. 3,636,679 and McCombs U.S. Pat. No. 3,738,087, both incorporated herein to the extent pertinent. Fir purposes of the invention this type of system may be broadly described in process terms as an adiabatic pressure swing process for gas separation by selectively adsorbing at least one gas component in at least two adsorbent beds and withdrawing one component-depleted product gas therefrom at specified average product purity and variable pressure in a repetitive cycle by introducing feed gas at progressively higher pressure to the inlet end of a first adsorbent bed thereby progressively increasing the pressure therein and discharging one component-depleted product gas from the discharge end with a one component mass transfer front being initially established at the inlet end and moving toward the discharge end during cycle steps including cocurrently depressurizing the first bed and terminating such cocurrent depressurization when the first bed is at lower pressure. One part of the first bed gas is withdrawn during the cocurrent de-pressurization as product gas and another part also withdrawn during the cocurrent depressuriation is returned for partial repressurization of another previously purged adsorbent bed. Waste gas is released fron the cucurrently depressurized first bed inlet end thereby countercurrently depressurizing same to a lowest pressure. One component-depleted gas from another adsorbent bed discharge end is introduced to the depressuized first bed discharge end as purge gas for desorption of the one component adsorbate and the one component-containing purge gas is discharged from the first bed inlet end. Then one-component depleted gas fron an other-than-first bed discharge end is introduced at above the lowest pressure to the purged first bed for partial repressurization thereof orior to reintroducing progressively higher pressure feed gas to the first bed inlet end.
Unless adjustments are made when the product gas withdrawl rate fron such a system drops substantially below the maximum rate, the purge and repressurization requirements of adsorption beds not delivering one-component depleted gas are satisfied before the cocurrently depressurizing bed has reached the lower pressure at which the cycle would normally be advanced to the next step. If, however, the cycle is advanced at this point of time the gas rejected during the countercurrent depressurization step will contain one component-depleted gas which would be delivered as product gas at maximum withdrawl rate. The quantity of product gas withdrawn per cycle from adiabatic pressure swing systems operating on a completely timed cycle is directly proportional to the rate of product withdrawl from the system, i.e. the quantity of product gas withdrawn per cycle is halved when the product withdrawl rate is halved so that the percent recovery is reduced by one half.
Such inefficient use of the system product capacity is in part avoided by practicing a control method described in the aforementioned Batta U.S. Pat. No. 3,636,679. The cycle is advanced when the gas pressure of the cocurrently depressurizing bed delivering product gas has experienced a pressure reduction commensurate with the delivery of its fully rated quantity of product gas. By way of illustration and referring to FIG. 4, the cocurrent depressurization step of bed A is terminated when the bed pressure declines to 16 psigs and beds B and C have previously been isolated between the points at which they complete their repressurization and purge steps respectively and point at which the bed A cocurrent depressurization terminal pressure is reached. In this manner the feed gas compresor may be "unloaded" or taken out of sevice when bed B reaches 40 psig and loaded again when all three beds are advanced fron step 4 to step 5. This represents a feed gas compression power saving and reciprocating-type compresors are well suited for such intermittent unloading-loading type of service.
Those skilled in the adsorption art recognize that the length of a selectively adsorbed one component mass transfer zone in selective adsorption systems is a direct function of the gas flow rate through the bed. This relationship is for example described in the book "Mass Transfer Operation " by Robert E. Treybal, McGraw-Hill Company, New York, 1955, page 490. As used herein the length of the mass transfer zone refers to the longitudinal portion of the adsorbent bed in the direction of feed gas flow, between the leading and trailing edges of the moving mass transfer front as described more completely in Kiyonaga U.S. Pat. No. 3,176,444.
Based on the aforedescribed state-of-art, one would logically expect that when a variable adsorption pressure type pressure swing adsorption system is "turned down" and controlled in the Batta manner, the one component mass transfer zone (which was contained in the bed at maximum product flow rate) will also be fully contained at the reduced flow rate.
Contrary to these expectations it has been discovered that when the Batta cocurrent depressurization pressure control method is practiced during turndown, the product recovery is substantially adversely affected and the product purity undesirable increases. By way of example, in a three bed adiabatic pressure swing adsorption system receiving air as feed gas and discharging 70% oxygen as the product gas, the oxygen recovery declines fron 100% to 80% when the oxygen production rate is turned down by 36%. At the same time the purity of the oxygen product gas increases to about 87%. It will be readily apparent that the system is operating substantially less efficiently under turndown conditions.
An object of the invention is to provide an improved control method of and apparatus for adiabatic pressure swing adsorption systems of the increasing adsorption pressure type during periods of reduced product withdrawl rate.
A more specific object of the invention is to provide a control system which permits reduction of the product withdrawl rate without simultaneously reducing the product recovery to the extent experienced by the prior art.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.